The present invention relates to a cellulose hydrate-based product which is permanently plasticized by a primary plasticizer and optionally contains an additional secondary plasticizer. The product is preferably a film, in particular a tube. The invention furthermore relates to a process for the production of this product and the use thereof in the form of a fiber-reinforced film.
It is known that cellulose-hydrate shaped products are produced by the viscose process. The alkaline cellulose xanthate solution, generally known as the viscose solution, is extruded, for example, through a ring- or slot-shaped die, coagulated as cellulose hydrate gel using an acidic solution and regenerated to form cellulose hydrate. The properties of the regenerated cellulose product can be varied through the composition of the viscose and by incorporating additives. In the production of fiber-reinforced tubes, a tube-shaped fibrous material is coated outside and inside with viscose and treated in an appropriate manner with coagulation and regeneration liquid.
It is known that cellulose hydrate products become increasingly brittle and hard on storage due to loss of water and plasticizer. This phenomenon is due to crystallization taking place in the cellulose material, whereby formation of hydrogen bonds between the individual cellulose molecule chains, and thus convergence and spatial localization of the molecules, occur in the cellulose structure. This crystallization and structural modification process results in impairment, increasing with time, of certain physical properties, in particular the expansion, strength and swelling value of the product. In addition, it causes considerable shrinkage of the product, so that, in the case of tubular packages based on cellulose, a considerable increase in pressure on the enclosed product is stated.
This structural modification process, which proceeds gradually in cellulose hydrate products, and the associated disadvantageous embrittlement of the products, can scarcely be prevented by adding water-soluble, so-called secondary plasticizers. Although secondary plasticizers, such as, for example, glycerol, glycol or polyglycol, are able to improve the plasticity and grip of the product, they tend to migrate out of the product or are quantitatively dissolved out, in particular during washing as is customary in sausage casings made from this material before filling with sausage meat. This occurs because secondary plasticizers are not bonded to the cellulose hydrate molecules by chemical bonding, but instead only by intermolecular forces.
The consequence is particularly severe embrittlement of the plasticizer-free dried cellulose material. Sausage casings having an internal barrier layer, whereby the moisture from the sausage meat cannot pass into the cellulose layer, become shock-sensitive and tend to tear along their entire length when the sausage is first cut. It is therefore customary to briefly moisten the sausage with cold water before the first cut if it has a sausage skin of this type. However, this measure is not able to prevent the sausage meat bulging or even being forced out from the cut surface by the high internal pressure.
In order to avoid these disadvantages, it has also already been described to improve the flexibility of cellulose hydrate by adding, as so called primary plasticizers, chemical substances which are capable of crosslinking. See, e.g., German Offenlegungsschrift No. 2,362,551 =U.S. Pat. No. 4,002,712, and German Offenlegungsschrift No. 2,654,417 =U.S. Pat. No. 4,198,325. Primary substances which permanently plasticize the cellulose hydrate are taken to mean compounds which, in contrast to secondary platicizers, cannot be extracted with water and are fixed in the product. However, the known primary plasticizers are not as yet a satisfactory solution to the problem. One disadvantage of the known, non-extractable plasticizers is that the cellulose material has to contain very large amounts of these compounds in order to achieve an optimum effect. Due to the attendant marked impairment in the mechanical properties of the cellulose material, this process has hitherto not become established in practice.
Previous attempts have therefore also been made to prevent these disadvantages by applying an elastomeric plastic coating on the outside of the cellulose-based sausage casing (European Patent No. 0,001,545 =U.S. Patent No. 4,287,217). However, this coating only has the optimum effect when it has a weight of at least 5-8 g/m.sup.2, i.e., when it is relatively thick. However, a weight per unit area of this magnitude should be avoided since a plastic coating of this type already forms a considerable outer barrier layer against water, so that problems occur on washing the sausage casing before the filling process, in particular when a barrier layer against water vapor and oxygen is provided on the inside of the casing. It has shown that coatings made from this elastomeric plastic tend to become detached from the cellulose surface when the sausage is boiled, so that an additional bonding layer is necessary to improve the adhesion. The application of this additional layer makes further process steps necessary and thus makes the product more expensive.
It is also known to employ the same outer coating in a considerably lower amount (80-500 mg/m.sup.2) in the case of casings without a barrier layer (European Patent Application No. 0,100,056 =U.S. Pat. No. 4,529,634). This coating is intended to prevent enzymatic degradation of the cellulose by cellulytic enzymes, as can occur on storage of uncooked and salami-type sausages. However, it has been found that this coating does not exhibit an adequate action. In larger, more effective amounts, undesired reduction in the water-vapor permeability occurs, which delays drying and impairs maturing of the sausage.